The question (and variants of it) is very broad and comes up a lot, so you should use the search box here and/or try math.stackexchange.com. Anyway, check out previous questions on MO such as this one (and various answers giving references): mathoverflow.net/questions/88017
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Jim HumphreysJan 17 '13 at 14:19

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See in particular Derek Holt's answer to the question linked to by Jim Humphreys.
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Alex B.Jan 17 '13 at 15:52